Question

Why does oxygen (O) not have a subscript/number?

Answer 1

Because we distinguish betrween oxygen, the atom, and oxygen, the element.

Explanation:

Elemental oxygen is a diatomic molecule, and we represent it as `O_2`, or as dioxygen. This is a different species from atomic oxygen, `O`, and we have to put in energy to make the transition to this species:

`O_2(g) + Delta rarr 2O(g)`